Thermistors are used for temperature measurement since they are essentially resistive elements which behave as variable resistors with heat, that is a resistor with a high-negative temperature coefficient of resistance. In some cases the resistance of a thermistor at room temperature may decrease by almost six percent of each degree rise in temperature. In temperature measurement applications, the thermistor's relatively large resistance change per degree change in temperature provides good accuracy and resolution. Typically, thermistors are connected in simple bridge circuits and, as will be appreciated, readily indicate very small temperature changes. This characteristic, however, has certain drawbacks in that a large temperature range will not produce an output in typical bridge circuit with a differential amplifier for the amplifier will saturate. The sensitivity which would be the percent change in resistance per degree should therefore not saturate or over-drive the associated circuitry. For example, in typical circuits as seen in the U.S. Pat. No. 3,947,656, it is very easy to saturate the differential operational amplifier in an integrated switch, such as the RCA CA3079; and as seen in this patent, the temperature range is limited.
Thermistors as currently manufactured have resistances at room temperature (25.degree. C.) that vary from 100 ohms to 30 megohms. Generally speaking, high resistance units above 100k are used for high temperatures, for example from 300.degree. F. to 600.degree. F., while the intermediate resistance units from 2k to 75k are used at intermediate temperatures of 150.degree. F. to 300.degree. F., and the low resistance units 100 to 1k are for measuring low temperatures from -100.degree. F. to 150.degree. F. It has been recognized that the maximum resistance of the thermistor at low temperatures must not be excessive as spurious signal pick-up can result, which then leads to the use of shielded lines. Further it has been recognized that the minimum resistance at high temperatures must not be too low, as this will result in a decrease in sensitivity. In designing standard bridge circuits for use with thermistors, the rule of thumb has been to select the series resistor and the opposite bridge leg resistor equal to the thermistor resistance at the mid point of the temperature range being measured and the adjacent bridge leg resistor selected at the thermistor's resistance at the temperature where bridge null is desired. It can be appreciated therefore, that there are a number of constraints placed on the bridge circuit alone that is used with the thermistor, plus the problem of selecting a suitable thermistor. The above criteria puts severe restraints on a circuit designer and it would be preferable to be able to utilize a single thermistor for a plurality of applications.